Currently available tests for tuberculosis infection are not satisfactory especially in remote areas or developing countries. Smear microscopy which is the primary tuberculosis (TB) diagnostic in much of the world requires training to be performed successfully and it is not always possible to find adequately trained personnel. Chest X-rays are not specific for TB and, of course, require special equipment.
The purified protein derivative (PPD) assay relies on a delayed hypersensitivity reaction to the infectious tuberculosis bacterium (Mtb) antigens that takes 48-72 hours to develop, and cannot distinguish active from latent infection. GeneXpert™ and other genetic based tests are expensive as they rely on complex equipment in clinical laboratory settings.
The status of tuberculosis in individuals who are infected varies. About two-thirds of people that are exposed to Mtb may never show any sign of infection and fail to develop a positive test such as PPD, and at least some of them develop transient T-cell interferon gamma responses to Mtb antigens. In a second group, the Mtb survive and multiply to elicit a long-lived adaptive immune response which is reflected in a positive PPD test. About 90% of these never develop active disease. This is the group of individuals that are latently infected. About 10% of these infected individuals develop active TB.
It is important to distinguish between these types of tuberculosis, as well as to identify all those who have been infected as the spread of the disease is caused by exposure to aerosols from infected individuals. Those with latent TB or who are resistant may cause infection in others. Thus, there is a need for a test that can readily detect not only whether the individual is infected, but also is able to determine the nature of the progress of the infection.
Napolitano, D. R., et al., Clinical and Vaccine Immunology (2008) 151:638-643, suggest the Mtb protein ornithine carbamoyl transferase, the product of MT_1694 gene, in urine, as a candidate indicator for detecting active T B. Kashino, S. S., et al., Clinical and Experimental Immunology (2008) 153:56-62, add the products of MT_1721, MT_2462 and MT_3444 as candidates.